1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly to an optical pickup device suitable for a compatible optical pickup device for emitting several kinds of laser beams having different wavelengths to a recording medium.
2. Description of the Related Art
Currently, various optical discs such as a compact disc (CD) and a digital versatile disc (DVD) have been commercialized and widely used. Further, next-generation DVD standardization for recording and reproducing information using a blue-violet laser beam has been recently proceeded. In the next-generation DVD, information is recorded and reproduced using the blue-violet laser beam having a wavelength of about 405 nm. When the wavelength of the laser beam shortens, a higher density can be obtained.
Therefore, when the variety of optical discs increases, development of a so-called compatible optical pickup device capable of performing recording and reproduction on different kinds of optical discs is desired. In order to irradiate an optical disc with laser beams having different wavelengths, it is possible to employ an arrangement in which semiconductor lasers that emit laser beams having different wavelengths are separately disposed in the optical pickup device. However, when such arrangement is employed, spaces for separately disposing the semiconductor lasers and optical elements for guiding the laser beams to an objective lens are required corresponding to each semiconductor laser. Consequently, the external dimensions of the optical pickup device become large and the number of parts increases. Thus, an arrangement in which a plurality of laser elements having different emitting wavelengths are provided all together in a single CAN package has been studied. According to such arrangement, a space for disposing the semiconductor lasers can be reduced and an optical system can be commonly used among the laser beams.
However, when the plurality of laser elements are provided in the single CAN package as described above, a deviation occurs in a direction between the optical axes of the laser beams according to arrangement gap between the respective laser elements. Therefore, when the optical axis of the optical system is aligned with the optical axis of a laser beam, the optical axes of other laser beams deviate from the optical axis of the optical system. In recording and reproduction using the other laser beams, there arises a problem in that aberration of laser beams on a recording medium occurs to cause deterioration of optical characteristics.
Therefore, according to a prior art described in JP06-131688 A, a birefringence element is disposed immediately after a semiconductor laser including several kinds of laser elements, and the optical axes of the laser beams are aligned with one another by the birefringence element to guide the laser beams to the optical system.
However, the prior art requires an additional birefringence element. In addition, it is necessary to form in advance the laser elements such that the polarization plane of the reference laser beam is orthogonal to the polarization plane of each of the other laser beams. However, it is hard to form laser elements in which polarization planes of laser beams are made different from one another. Also, since the birefringence element is expensive, a problem occurs in that a cost of the optical pickup device as a whole increases.
Such a problem can be solved by using a diffraction grating as an optical axis correcting element. When the diffraction grating is used as the optical axis correcting element, an increase in cost can be suppressed. In addition, it is unnecessary to form a corresponding laser element in an adjusted position in view of the polarization planes of the respective laser light beams having wavelengths unlike JP 06-131688 A.
However, when the diffraction grating is used as the optical axis correcting element, a problem in which the power of each of the laser light beams attenuates occurs because of a relationship between each wavelength and diffraction efficiency.
The diffraction action made by the diffraction grating depends on n-times (n: natural number) a wavelength, so the same diffraction action is applied to laser light beams having different wavelengths in which a wavelength is equal to or nearly equal to n-times another wavelength. For example, in a compatible optical pickup device for CD, DVD, and next-generation DVD, a wavelength of a laser light beam for CD (infrared color: about 780 nm in wavelength) is nearly equal to two times a wavelength of a laser light beam for next-generation DVD (blue color: about 405 nm in wavelength). Therefore, it is difficult to effectively apply the diffraction action to only any one of the laser light beam for CD and the laser light beam for next-generation DVD. Thus, it is difficult to effectively align an optical axis of the laser light beam for CD with an optical axis of the laser light beam for next-generation DVD by using the diffraction grating.
In contrast to this, while the optical axis of the laser light beam for CD is not aligned with the optical axis of the laser light beam for next-generation DVD, when the optical axis of any one of the laser light beams is aligned with an optical axis of an optical system, a problem in which the optical properties of the other of the laser light beams deteriorate occurs. This deterioration becomes significantly as the amount of optical axis deviation between the laser light beams increases. In particular, when the optical axis deviation is caused on a photo detector, a reproduction signal and various error signals cannot be smoothly led. In such a case, the photo detector is required for each of the laser light beams. If so, it is necessary to dispose, for example, an additional optical part for beam separation. This causes an increase in the number of parts and the complication of a structure.